This study is a follow up to the study ACTG 175 which compared monotherapy (AZT or ddI) versus nucleoside analog combination therapy (AZT/ddI or AZT/ddC). This new study compares the efficacy of continuing AZT/ddI or AZT/ddC versus adding 3TC or switching to AZT/3TC. Enrollment is open to patients who were either originally randomized to combination therapy of were crossed over to this treatment after reaching a study end-point and who have remained on the same treatment until enrollment into this study. This study will investigate if adding 3TC to a combination of AZT plus ddC (making a three-drug combination) is better than, worse than, or the same as the two-drug combination (AZT and ddI, or AZT and ddC) or the combination of AZT plus 3TC. The investigators will attempt to determine the influence of virus load, CD4+ cell counts, biologic phenotype, and presence of symptomatic HIV disease on the response to a new therapeutic regimen, the kinetics of viral replication, and the rate of emergence of the 3TC associated resistance mutation at codon 184.